The invention relates to a detector for the temporally resolved recording of detection events. The invention furthermore relates to an imaging device comprising such a detector. The invention also relates to a method for the temporally resolved recording of detection events.
Detectors for the temporally resolved recording of detection events are used when the time information is important for the evaluation of detector data or else when detection events which belong to one another in time terms are to be found. This is the case for example in Positron Emission Tomography (PET). Such PET devices are used to measure the annihilation quantum pairs which occur in matter in the case of positron emission. Positron emitters (e.g. fluoro compounds comprising the fluorine isotope with atomic number 18) are for instance injected into a patient and distribute in a specific manner in the body of the patient, depending on the type of chemical compound containing the positron emitter. In the case of positron emission, the positron will typically annihilate with an electron after a few tenths of a millimeter (mm) and two gamma quanta each of 511 kiloelectronvolts (keV) energy are irradiated in essentially opposite directions. The quanta are recorded by means of a detector arranged around the patient, said detector forming part of the PET device. Two detector events are assigned to a single annihilation process if they lie within a coincidence time window having a length of a few nanoseconds (ns). The distribution of the injected positron emitter in the body of the patient can be reconstructed by means of the measured coincident events.
The quality of the measurement, the associated reconstruction of the distribution of the positron emitter and the meaningfulness of the diagnosis that can be subsequently derived from the measured distribution can be improved if the point of annihilation can be determined (approximately). For this purpose, temporal resolutions of up to 0.1 ns or even shorter times are required. An example of such a method is “Time of Flight” PET (TOF PET).
US patent application US 2001/0017352 discloses a device for improving the image quality in positron emission tomography. This device comprises photomultipliers which are in each case connected to an amplifier which is followed by an analog/digital converter (ADC). By means of a central digital clock running at 2 ns cycles, so-called time stamps are added to the digital signal of a detection event following the ADC for the purpose of further evaluation.
In order to ensure the high temporal resolution, which allows a spatial assignment of detection events to an annihilation point, high frequency time information or trigger signals in the Gigahertz (GHz) range (about 1-10 GHz) must be transmitted over the entire detector to the individual detector channels or from the individual channels to a central time acquisition unit. In order to ensure highly accurate, precise and loss-less transmission of such frequencies within the detector arrangement, there is a need for high technical and financial outlay. By way of example, rapid digital memories are required. In addition, the high frequency signals from other components, for instance the amplifiers, may cause interference.